<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>盒子内垂直水平居中</title>
  <style>
    div{
      width: 100%;
      height: 100vh;
      background-color: aqua;
    }

    span{
      display: flex;
      width: 50%;
      height: 50%;
      margin: auto;
      background-color: gray;
      position: relative;
      top: 50%;
      transform: translate(0,-50%);
      /*span中的内容水平方向对齐项目*/
      justify-content: center;
      /*span中的内容垂直方向对齐项目*/
      align-items: center;
    }
  </style>
</head>
<body>
<div>
  <span>13444546</span>
</div>
</body>
</html>